In cardiac surgery, systemic hypothermia is used with cardiopulmonary bypass for myocardial protection and preservation of function. Marked changes occur in organ perfusion and metabolism with hypothermia. The hypothesis of this experimental protocol is that arterial pH is a major determinant of systemic oxygen consumption and anaerobic metabolism during hypothermia cardiopulmonary bypass and of the total body carbon dioxide content in the post-bypass period. The present experimental protocol was designed to measure systemic oxygen consumption, and the metabolic acid-base status of patients undergoing hypothermic cardiopulmonary bypass for cardiac surgical procedures. Systemic and venous blood samples for blood gases and lactate concentrations and cardiac output or cardiopulmonary bypass flow were measured prior to, during, and after cardiopulmonary bypass in forty patients. The patients were randomly assigned to one of two groups. The blood pH during hypothermia was kept constant at 7.4 in one group (pH-stat). In the other, the pH was increased 0.015 pH units for each one degree decrease in temperature (alpha-stat). Results show that a uniformly low operative mortality was obtained with either pH scheme. The data on oxygen consumption and lactate production is in the process of analysis.